The present invention pertains to polyphase DC motors, and particularly to three-phase DC motors. Brushless, sensorless-type DC motors are used for rotating data media, such as found in computer-related applications, including hard disk drives, CD-ROM drives, floppy disks, VCRs (video cassette recorders), and the like. In computer applications, three-phase, brushless, sensorless DC motors are becoming more popular, due to their reliability, low weight and accuracy.
Motors of this type can typically be thought of as having a stator with three coils connected in a "Y" configuration, although a larger number of stator coils are usually employed with multiple motor poles. Typically, in such applications, 8-pole motors are used, which have four electrical cycles per revolution of the rotor. The stator coils, however, can be analyzed in terms of three "Y" connected coils. In operation, the coils are energized in sequences in which a current path is established through two coils of the "Y" with the third coil left floating in bipolar applications. The sequences are arranged so that as the current paths are changed, or "commutated", one of the coils of the current path is switched to float and the previously floating coil is switched into the current path. The sequence is defined such that when the floating coil is switched into the current path, current will flow in the same direction in the coil which was included in the prior current path. In this manner, six commutation sequences are defined for each electrical cycle in a three-phase motor.
Each coil is connected to a high side driver and a low side driver. When the high side driver is enabled, that coil will be operatively connected to a voltage supply for providing current through the coils. The low side driver selectively couples a coil to ground. Hence, the two coils can be chosen by appropriately enabling and disabling the high side and low side drivers. It is advantageous to enable the high side driver to quickly turn-on once its respective coil has been selected. However, a fast transition at the output of the driver will result in EMI (electromagnetic interference) and noise, which may create errors in the signals from the reading head of a hard drive. In the past, a significant amount of noise could be tolerated in the reading circuitry. However, with higher density hard drives and other devices, the circuitry is much more sensitive to electromagnetic interference and noise. Consequently, the use of prior art motors, and particularly the circuitry to drive the coils of a DC motor, can negatively effect performance of the hard drive or similarly sensitive device.
Therefore, a need has arisen in the industry for a motor with driving circuitry with reduced EMI and noise, while maintaining satisfactory speed performance.